refactor: make Settings UI the single source of truth for API provider

Remove legacy env-var-based provider/mode detection that caused misleading
UI badges (e.g., GLM badge showing when Settings was set to Claude).

Key changes:
- Remove _is_glm_mode() and _is_ollama_mode() env-var sniffing functions
  from server/routers/settings.py; derive glm_mode/ollama_mode purely from
  the api_provider setting
- Remove `import os` from settings router (no longer needed)
- Update schema comments to reflect settings-based derivation
- Remove "(configured via .env)" from badge tooltips in App.tsx
- Remove Kimi/GLM/Ollama/Playwright-headless sections from .env.example;
  add note pointing to Settings UI
- Update CLAUDE.md and README.md documentation to reference Settings UI
  for alternative provider configuration
- Update model IDs from claude-opus-4-5-20251101 to claude-opus-4-6
  across registry, client, chat sessions, tests, and UI defaults
- Add LEGACY_MODEL_MAP with auto-migration in get_all_settings()
- Show model ID subtitle in SettingsModal model selector
- Add Vertex passthrough test for claude-opus-4-6 (no date suffix)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/App.tsx

@@ -319,7 +319,7 @@ function App() {
                   {settings?.ollama_mode && (
                     <div
                       className="flex items-center gap-1.5 px-2 py-1 bg-card rounded border-2 border-border shadow-sm"
-                      title="Using Ollama local models (configured via .env)"
+                      title="Using Ollama local models"
                     >
                       <img src="/ollama.png" alt="Ollama" className="w-5 h-5" />
                       <span className="text-xs font-bold text-foreground">Ollama</span>
@@ -330,7 +330,7 @@ function App() {
                   {settings?.glm_mode && (
                     <Badge
                       className="bg-purple-500 text-white hover:bg-purple-600"
-                      title="Using GLM API (configured via .env)"
+                      title="Using GLM API"
                     >
                       GLM
                     </Badge>